A seat belt apparatus installed in an automotive vehicle for the purpose of restraining the body of a passenger in a seat is provided with a pretensioner, which is a component of the apparatus actuated in the event of an emergency to tension the belt. Several types of pretensioners are available, such as one which actuates a retractor that is a component of the seat belt apparatus, one which tensions the belt directly, and one which pulls down on a buckle to which the belt is attached. Some pretensioners use an explosive mixture as a driving source for achieving actuation, while others use a spring as the driving source.
In those pretensioners that use an explosive mixture as the driving source, an electrical trigger is required for actuation, and problems arise in terms of high cost and of disposal of the vehicle. Accordingly, various pretensioners have been proposed in which use is made of a spring that can be arranged to operate in a purely mechanical manner.
However, the driving source of a pretensioner must have a sufficient stroke and actuating force. Accordingly, in a case where a spring is adopted as the driving source, using a strong spring requires a large load to set the spring, and a considerable actuating force is necessary to release the spring. In a conventional seat belt apparatus having a static sensor serving as a trigger for actuating a retractor and sensing acceleration in the form of movement of a mass by inertia, it is difficult to reliably actuate a trigger which releases the locked spring.
Accordingly, the mechanism for actuating a pretensioner using a driving source, such as a spring, that needs a large preload requires a trigger actuated by a small force and an acceleration sensor capable of outputting a large mechanical signal.